It has been long known that the mitotic processes of cancer and the forming and dividing of the chromosomes were riotous and irregular, like the rest of its growth. But it was reserved for these investigators to discover the extraordinary fact that the majority of dividing and multiplying cancer-cells had, instead of the normal number of chromosomes, exactly half the quota. In other words, they had resumed the powers of the germ, or sexual, cells from which the entire body was originally built up, and were, like them, capable of an indefinite amount of multiplication and reproduction. How extraordinary and limitless this power is may be seen from the fact that a little group of cancer-cells grafted into a mouse to produce a Jensen tumor, from which a graft is again taken and transplanted into another mouse, and so on, is capable, in a comparatively few generations, of producing cancerous masses a thousand times the weight of the original mouse in which the tumor started! In short, cancer-cells are obviously a small, isolated group of the body-cells, which in a ghastly fashion have found the fountain of perpetual youth, and can ride through and over the law-abiding citizens of the body-state with the primitive vigor of the dawn of life. This brings us to the most practical and important questions of the problem: What are the influences which condition this isolation and outlawry of the cells? What can we do to prevent or suppress the rebellion? To the first of these science can only return a tentative and approximate answer. The subject is beset with difficulties, chief among which is the fact that we are unable to produce the disease with certainty in animals, with the single exception of the Jensen's tumors in mice referred to, nor is it transferred from one human being to another, so that we can make even an approximate guess at the precise time at, or conditions under, which the process began. Many theories have been advanced, but most investigators who have studied the problem in a broad-minded spirit are coming gradually to agree to this extent:--